This invention is related to efficient inorganic borophosphate phosphors which can applied in various technical applications such as fluorescent lamps, colored light or white light emitting diodes, and other devices where phosphors are used to convert especially near UV radiation into the visible light. Further, this invention is related to light sources comprising the efficient borophosphate phosphor.
In the solid-state lighting industry, the wavelength conversion phosphor materials play a crucial role as they once did in fluorescent lamps. White LED lighting within a phosphor converted-LED (pc-LED) system can be realized by several approaches: The first approach is to combine the InGaN based blue-LED chip (emitting blue light between 455 nm and 465 nm with a yellow phosphor, i.e., YAG:Ce3+ or EA2SiO4:Eu2+ based materials. However, the white LED provided by this well known and established method has the disadvantages of poor color rendering when used for general lighting and small color gamut when used for backlighting. The second one is to combine a blue-LED chip with a green-emitting phosphor (λmax˜530 nm) and a red-emitting phosphor (λmax>600 nm) instead of the single yellow-emitting phosphor. The two phosphors absorb the blue light from the InGaN chip and convert it into green and red light and then by color mixing the white light is generated with higher color rendering and larger color gamut. But in both methods the final color temperature (CCT) and color coordinate of the pc-LED strongly depends on the emission characteristics of the blue-LED chips. As a consequence, in order to get a similar CCT only a part of available blue-LED chips can be used. The third one is to use a near UV-LED chip plus blue, green, and red emitting phosphor. In comparison with the former two ways, the third one provides improved color rendering and a wide range of color temperatures as well as an independent color coordinate. The disadvantage of this technical solution is the color shift during the life cycle resulting from the different aging rate of the three different phosphors.
There are also attempts to provide phosphors that are excitable by a radiation source of the near UV and emit the visible light, especially, a single phosphor which can emit white light without the request to combine with some other phosphors. Its emission spectrum is composed of the three primary colors (blue, green, and red) and covers the whole visible range from 400 nm to 700 nm. Subsequently, some of these attempts are cited:
In the article of Park et al. in Appl. Phys. Lett. 82 (2004) pages 2931-2933; Solid state comm. 136 (2005) 504, a phosphor of the general formula: EA3MgSi2O8:Eu2+, Mn2+ (EA=Sr, Ba) is presented. This phosphor shows three emission bands peaking at 422 nm, 505 nm and 620 nm. The 442 nm and 505 nm emissions originate from Eu2+, while the 620 nm emission originates from Mn2+ ions. The fabricated white light emitting light diode integrating 400-nm-emitted chip with EA3MgSi2O8:Eu2+, Mn2+ (EA=Sr, Ba) phosphor shows warm white light and higher color rendering index and higher color stability against input power in comparison with a commercial blue-pumped YAG:Ce3+. However, this white LED has a low luminous efficiency and a poor long-term stability.
In J. Electrochem. Soc. 155 (2008) pages J 193-J 197; Electrochem. Solid state lett. 11 [2] (2008) E1, white light emitting phosphor has been proven to be a mixture of EA3MgSi2O8:Eu2+, Mn2+ (EA=Sr, Ba) and EA2SiO4:Eu2+ (EA=Ba, Sr). In fact, the emission band peaking at 505 nm originates from EA2SiO4:Eu2+ (EA=Ba, Sr) instead of EA3MgSi2O8:Eu2+, Mn2+ (EA=Sr, Ba).
In the article by Lakshminarasimhan et al. in J. electrochem. Soc. 152 [9] (2005) H152, systems of the formula Sr2SiO4:Eu,Ce are suggested. This system exhibits a low efficiency and poor stability.
In the article by Chang et al. in Appl. Phys. Lett. 90 (2007) 161901) systems of the formula Ca2MgSi2O7:Eu,Mn. This system also exhibits a low efficiency and poor stability.
In the article of J. Liu et al. in Adv. Mater. 17 (2005) pages 2974-2978, a single phosphor is presented that can emit the blue (445 nm), green (515 nm), and red (624 nm) light simultaneously. The CIE coordinates are located at (0.31, 0.34), which is very close to (0.33, 0.33) of the standard white emission. The host lattice is an organic compound, which is not very stable under high temperature.
WO 2006/111568 A2 shows white light emitting non-stoichiometric compounds having a not fixed composition.
Recently, in the search for new functional materials, borophosphate, which contain both the borate group and the phosphate group as basic structural units, has also drawn attention. In the last couple of years many borophosphates were synthesized and structurally characterized. As far as luminescence is concerned, most of work has been focused on studying the luminescence properties of rare earth ions in MBPO5 (M=Ca, Sr, Ba), (Ba, Sr)3BP3O12, and (Ba, Sr)6BP5O20Ba3BPO7 host lattices.
DE 1 927 455 shows borate phosphate phosphors like Ba0.995Eu0.005BPO5 and Sr0.99Eu0.01BPO5 that are used in low-pressure mercury discharge lamps. These Eu2+-activated MBPO5 phosphors show a broad emission band in the UV to blue range with a maximum, dependent on the alkaline earth metals present, at 385 nm to 400 nm.
DE 29 00 989 A1 shows borate phosphate phosphors of the general formula Ba3-pEupBP3O12 for the usage in low-pressure mercury discharge lamps. These phosphors show a greenish emission with the maximum in the wavelength range of 490 nm to 520 nm.
In Appl. Phys. B 86 (2007), Pages 647-651, Eu2+-activated (Ba,Sr)6BP5O20 phosphors are shown that exhibit bluish green emission in the wavelength range of 470 nm to 510 nm.
A new kind of borophosphate, i.e. KMBP2O8 (M=Ba, Sr), was discovered by Zhao et al. [Inorg. Chem. 48 (2009) pages 6623-6629] in 2009. Until now, the luminescence properties of the rare earth ions in these host lattices have never been reported. The main structure features of this host lattice are similar to those of the other borophosphate compounds, i.e. a network of PO4 tetrahedron and BO3 triangles/BO4 tetrahedrons.
WO 2009/036425 A1 shows a phosphor blend for a compact fluorescent lamp comprising LaPO4:Ce3+, Tb3+; Y2O3:Eu3+; Sr6BP5O20:Eu2+ and Mg4GeO5.5F:Mn4+.